All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae), currant-lettuce aphid (Nasonovia ribis-nigri) and lettuce mosaic virus (LMV). These diseases result in millions of dollars of lost lettuce crop throughout the world every year.
Downy mildew (Bremia lactucae) is highly destructive of lettuce that is grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, B1:17, B1:18, B1:21, B1:22, B1:23, B1:24, B1:25, B1:26, B1:27 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151), Ca-V, Ca-VI, Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.” In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, Calif., pp. 55-68). Also new strains appear, which are overcoming resistance gene combinations in present varieties. An example is strain FR10.021 which is overcoming the Bremia-resistance in the variety “Expedition”. New strains can be described by a sextetcode based on resistance test results on a set of 25 publicly available lettuce lines (IBEB press release May 2010. FR10.021 has sextetcode 63-31-62-03.
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Lettuce mosaic virus (LMV) mainly infects lettuce seeds, which is the primary way that the virus is introduced to lettuce in the fields, but also can infect numerous crops and weeds, thereby creating reservoirs of the virus. LMV also can be vectored by aphids, which spread the virus within a lettuce field and introduce it into lettuce fields from infected weeds and crops outside the field.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribis-nigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as deep in the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
Symptoms of lettuce mosaic virus vary greatly. Leaves of plants that are infected at a young stage are stunted, deformed, and (in some varieties) show a mosaic or mottling pattern. Such plants rarely grow to full size; head lettuce varieties infected early fail to form heads. Plants that are infected later in the growth cycle show a different set of symptoms. These plants may reach full size, but the older outer leaves turn yellow, twisted, and otherwise are deformed. On head lettuce, the wrapper leaves often will curve back away from the head and developing heads may be deformed. In some cases brown, necrotic flecks occur on the wrapper leaves.
Although several known lettuce cultivars exhibit resistance against disease, irrespective of lettuce type, many lettuce cultivars affected produce large leaves that, when cut to smaller size pieces generally result in a lot of cut surface resulting in a diminished shelf life with respect to wound-induced discolouration of these cut surfaces. Light green or ‘blond’ varieties are considered better tasting by consumers, due to their association with leaves from the heart of a lettuce plant, which are milder tasting as well as paler coloured than outer leaves.
Although several known lettuce cultivars can be harvested mechanically at young plant, i.e. babyleaf stage, no pest and disease resistant lettuce cultivars exist that can be harvested mechanically at mature stage and still provide leaf pieces that are of small, directly edible size and have an attractive green colour. Mechanical harvesting saves labour cost and improves labour conditions in comparison with commonly applied hand-harvesting methods.
There exists a need, therefore, for an mechanically harvestable lettuce variety which exhibits a combination of resistance against downy mildew (Bremia lactucae), currant-lettuce aphid (Nasonovia ribis-nigri) and Lettuce Mosaic Virus (LMV).
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.